A semiconductor element, an organic EL element, or the like is manufactured by performing a processing such as, for example, etching, CVD, or sputtering, on a processing target substrate. In the related art, a plasma processing apparatus using a radial line slot antenna has been known as a plasma processing apparatus that performs a predetermined plasma processing on a processing target object such as, for example, a semiconductor wafer. The radial line slot antenna is disposed on the top of a dielectric window disposed in an opening of a ceiling of a processing vessel, in a state in which a slow wave plate is disposed on the top of a slot plate having a plurality of slots. The radial line slot antenna is connected to a coaxial waveguide at the central portion. With this configuration, microwaves generated by a microwave generation unit are radially transmitted by the slow wave plate via the coaxial waveguide, circularly polarized waves are generated by the slot plate, and then the microwaves are radiated into the processing vessel from the slot plate through the dielectric window. Further, high density plasma having low electron temperature may be generated under low pressure within the processing vessel by the microwaves, and a plasma processing such as, for example, film forming or etching, is performed by the generated plasma. Thus, the method of using the microwaves as a plasma source is advantageous in that the plasma is maintained at a low electron temperature, and as a result, damage to the processing target object is reduced.
In the related art, as a method of introducing microwaves, there is, for example, a method of arranging an annular waveguide or introducing microwaves from a cavity resonance portion through a slit. In addition, the above-described method of introducing microwaves from a radial line slot antenna may control plasma distribution by changing an antenna pattern, and form plasma excellent in uniformity of plasma density by optimizing the shape of the slot antenna. Further, the uniformity may be further improved by bringing a microwave transmitting dielectric and the antenna into close contact with each other. Further, a high-density and stable plasma may be formed by forming a recess in the dielectric, through which the microwaves are introduced, so as to optimize the slot pattern.
In the plasma processing apparatus, a uniform processing throughout the entire surface of the processing target object is required. In order to improve the in-plane uniformity in the plasma processing, it is required to stably form a more uniform plasma within the processing vessel where the plasma processing is performed.
Since very high electric power of about 1 KW to 5 KW is required for microwaves for generating plasma, magnetron oscillation tubes have been used in the related art. However, the oscillation frequencies of the magnetron oscillation tubes are different per each solid, and for example, in the case of 2.45 GHz, the frequency deviations of ±10 MHz or more exist. The frequencies also vary depending on the state of a plasma load, a power, or the like.
In a plasma processing apparatus using microwaves as a plasma source, the influence of the frequency of microwaves is high, and as the frequency is changed, the distribution and density of plasma are changed, and a uniform plasma processing may not be performed. In order to enhance the stability of magnetron frequencies, for example, a method of controlling injected synchronous signals has been studied and improved in the related art, but it was difficult to completely fix the frequencies.
Further, in the relationship between plasma and microwave absorption, a phenomenon is known in which an absorption peak at which microwaves are absorbed to the plasma exists at a specific frequency. However, since the frequency at which the absorption peak occurs changes every moment, it is very difficult to always maintain a state in which the microwaves are absorbed. Moreover, the frequencies of microwaves are influenced by frequency characteristics according to various shapes of, for example, a plasma processing vessel and an antenna waveguide. In addition, it has been difficult to directly monitor the state of plasma density in the related art.
For example, Patent Document 1 discloses that the frequencies of microwaves in a broadband may be made uniform by frequency-modulating carrier signals in a plasma processing apparatus.
In addition, Patent Document 2 discloses a method of generating microwaves efficiently and stably using a solid oscillator.